Three patients with primary pulmonary hypertension have been studied in the basal state and after administration of nitroprusside, nitroglycerin, isoproterenol and tolazoline. One patient had a marked decrease in total pulmonary resistance with all drugs. The response of the pulmonary vascular bed to drugs in these patients appears unpredictable and in particular, isoproterenol may be hazardous in some patients by increasing cardiac output and pulmonary pressure.